The present invention relates to fluid controllers of the type used to control the flow of fluid from a source of pressurized fluid to a fluid pressure actuated device such as a steering cylinder which comprises part of a full-fluid-linked steering system. More particularly, the present invention relates to such a steering system including an auxiliary, parallel fluid path and a logic control system for controlling such parallel fluid path.
Although the present invention may be used in connection with fluid controllers of many types, and having various applications, it is especially advantageous when used with a fluid controller of a type used in full-fluid-linked steering systems, and will be described in connection therewith.
A typical fluid controller of the type to which the present invention relates includes a housing which defines various fluid ports, and further includes a fluid meter, a valve means, and an arrangement for imparting follow-up movement to the valve means, in response to the flow of fluid through the fluid meter. The flow through the controller valve means is directly proportional to the area of the variable flow control orifices in the main fluid path, the area of the flow control orifices in turn being proportional to the rate at which the steering wheel is rotated.
A typical example of a vehicle which utilizes a fluid controller of the type to which this invention relates would be an agricultural tractor or combine. There is growing interest in being able to steer such vehicles by means of an electro-hydraulic steering system, and preferably, one which is "closed loop", i.e., one in which there is continuous correction of any "error" between the position of the steered wheels and the position of the steering wheel.
Prior to the present invention, in order to provide a vehicle with both conventional rotary input, full-fluid-linked steering, and some sort of closed loop, electro-hydraulic control, would have required that the vehicle have both a conventional fluid controller and a separate, parallel control system, operable in response to various signals, such as a steered wheel position signal and a steering wheel position signal. Although such systems have been generally known, at least in concept, there has been very limited commercial use of such systems.
One of the problems associated with such systems is the difficulty of coordinating operation of the conventional fluid controller with that of the parallel, electro-hydraulic valve. For example, under certain circumstances, it is desirable for the fluid controller to override the electro-hydraulic valve. It is also desirable, and on many vehicle applications it is absolutely necessary, to be able to manually steer the vehicle with the fluid controller, thus further complicating the coordination between the fluid controller and the electro-hydraulic valve.
Another problem area associated with systems including both fluid controllers and parallel electro-hydraulic valves relates to a series of steering performance criteria which are of particular concern to the vehicle operator. These performance criteria are of special concern in such systems where a portion of the flow to the steering cylinder is independent of the fluid controller and the fluid meter which is typically included in such controllers. For example, there should be no "wander" or "drift" in the steering system, i.e., the steered wheels should not move whenever the operator is not rotating the steering wheel. As another example, there should be proper "knob" control, i.e., whenever the vehicle operator returns a steering wheel knob to a certain position, the steered wheels should always return to a corresponding position. As a final example, it is desirable that corrections made by means of the parallel, electro-hydraulic valve not be readily apparent to the vehicle operator, such that the vehicle operator has the perception of not being fully in control of the steering system.